## Description

https://leetcode.com/problems/longest-substring-without-repeating-characters/

Given a string, find the length of the **longest substring** without repeating characters.

**Example 1:**

Input:"abcabcbb"Output:3Explanation:The answer is`"abc"`

, with the length of 3.

**Example 2:**

Input:"bbbbb"Output:1Explanation:The answer is`"b"`

, with the length of 1.

**Example 3:**

Input:"pwwkew"Output:3Explanation:The answer is`"wke"`

, with the length of 3. Note that the answer must be asubstring,`"pwke"`

is asubsequenceand not a substring.

## Explanation

We use two pointers technique to solve the problem. One slow pointer **i**, one fast pointer **j**.

We also add a HashSet to store the characters which have been visited by **j** pointer to help detect repeating characters.

We keep moving **j** pointer right further.

- If current
**s.charAt(j)**character is not in the HashSet, we add the character to the HashSet and keep moving**j**further. - If current
**s.charAt(j)**character is in the HashSet, we remove the character i is visiting and move**i**forward. At this point, we found the maximum size of substrings without duplicate characters start with index**i**. We move**i**pointer one step further.

When **j **pointer iterates all the characters of the string, we get the max length of the longest substring without repeating characters.

## Java Solution

```
class Solution {
public int lengthOfLongestSubstring(String s) {
int maxLength = 0;
HashSet<Character> set = new HashSet<>();
int i = 0;
int j = 0;
while (j < s.length()) {
if (!set.contains(s.charAt(j))) {
set.add(s.charAt(j));
j++;
maxLength = Math.max(maxLength, j - i);
} else {
set.remove(s.charAt(i));
i++;
}
}
return maxLength;
}
}
```

## Python Solution

```
class Solution:
def lengthOfLongestSubstring(self, s: str) -> int:
longest = 0
visited = set()
i = 0
j = 0
while j < len(s):
if s[j] not in visited:
visited.add(s[j])
else:
longest = max(longest, len(visited))
visited = set()
i += 1
j = i
visited.add(s[j])
j += 1
return max(longest, len(visited))
```

- Time complexity: O(n).
- Space complexity: O(m). m is the size of the charset.

Hello There,

If we consider this use-case, “abcddefghijal” – for this string, shouldn’t the output be 6 and not 9? i.e. (efghij) and not (defghijal) as d and a are repeating. Please correct me if I am wrong. Thanks.

This is incorrect

sorry, the solution is correct… i overlooked

lol, np.. thank you for visiting 🙂

Your answer is incorrect. What is the output for “abcabcbb” ?

The output for “abcabcbb” is 3. It is the correct answer. Which part you think is incorrect?

Print hashset before u return and it is showing wrong result.